The study was initiated to determine the molecular weights of the genomic RNA's of type B and C retraviruses by electron microscopy and polyacrylamide gel electrophoresis. Visualization of the RNA's was achieved by using a modified Kleinschmidt-technique, permitting optimal extension of the molecules. Standardization of the method was accomplished. The linear density (molecular weight/unit length) calculated as 0.9 x 10 to the 6th power/micron m was found to be constant for different control RNA's comprising a wide range of molecule sizes. The data obtained for the high molecular weight viral RNA's favor the assumption of two subunit molecules constituting the genome of certain retraviruses of type B and C. The study was initiated to characterize by electron microscopic means the morphologic aspects and the virus-host cell relationship of two new virus isolates obtained from M. cervicolor and M. caroli. Both isolates exhibit criteria which permit their classification in the family of retraviridae. A novel association of intracytoplasmic particles of type A, believed to be precursors of the mature virions, with certain cellular organelles was recognized. Evidence is presented of the association of viral precursors with the formation of the centriole. BIBLIOGRAPHIC REFERENCES: Ablashi, D. V., Loeb, W. F., Pearson, G., Valerio, M. G., Armstrong, G. R., Cicmanec, J. L., Heine, U. and Kingsbury, E. W.: Comparison of lymphomas induced in owl monkeys with Herpesvirus saimiri (HSV), heated HSV and HSV genome carrying cells. In: Monograph 5th International Conference on Multiple Primary Malignant Tumors (ed. L. Severi), Division of Cancer Research, University of Perugia, Perugia, Italy, 721-729, 1975. Ablashi, D. V., Oie, H. K., Armstrong, G. R., Didier-Fichet, M. L., Tronick, S. R., Heine, U. and Dalton, A. J.: Xenotropic Properties of an Isolate from Murine Rauscher Leukemia Virus in Primates. J. Med. Primatol. 5: 223-236, 1976.